I picked up three old PowerMacs over the weekend and the first two that I've tested are giving me pain. Both were very dirty, having being stored in a garage or similar for a prolonged time, so I cleaned them up and allowed them to acclimatise for three hours before trying to use them. Electrostatic precautions were taken during cleanup and both PRAM batteries were replaced (they read 0.00V and 0.01V!).
8200 (boringly stock specification): Power is successfully switched on from ADB keyboard but no boing. The standard Apple HDD performs motor start. I've cleaned the DIMM connectors with pencil eraser, applied contact cleaner to the DIMM slots, reset CUDA, removed new PRAM battery, disconnnected SCSI and floppy drive (power and data) but no success. For now I've put the system on one side until I have time to swap the board into a working 8200 system with reliable DIMMs etc.
9500 (modestly upgraded -- RAM and video): This booted first time but it took me a few more attempts to set the DIP switches on the Mac video/VGA converter. Once booted with video, the 9500 performed well for 20 minutes while I sniffed around the applications left installed on the boot drive. Shortly after, it powered off without warning; trying to restart generated the familiar click of a shorted power supply... I reseated the PCI video card and processor card, reset CUDA, removed new PRAM battery, disconnected all drives, allowed to rest unpowered for 20 hours etc. Still dead with immediate power supply shutdown when trying to boot (single click, no chirping).
I plugged the 8200 board into the 9500 chassis. In the 8200 chassis, the 8200 board had failed to boot but the psu switched on; in the 9500 chassis, the psu clicked off immediately.
So I swapped the 9500 psu for the similar one from the 8200. Using the standard Apple cpu card (604e?/120) and an untested XLR8 carrier G4/450, the psu comes on but no boing (ie no psu shutdown click). I have yet to perform more than a cursory clean up of the 9500 DIMMs but I'll clean them up more thoroughly shortly. However, my suspicion thus far is that when the psu went "pop", it blew the 9500 motherboard at the same time...
Any further thoughts? The 9500 motherboard is not common in the UK -- any board substitutions?